In recent years in the display field, multicolor emissive displays have been developed which are capable of multicolor or full-color display. In particular, considerable efforts have been made to develop high-resolution multicolor emissive EL displays, which can make effective use of the characteristics of organic EL elements. This is because in organic EL elements high current densities can be obtained at low applied voltages, and consequently high brightness and high emission efficiency can be realized.
Of these, color conversion-type organic EL displays have attracted attention. A color conversion-type organic EL display generally passes light emitted from an organic EL element through a color conversion layer and a color filter layer in sequence, to emit light having a prescribed wavelength to the outside. Here, a color conversion layer has the functions of absorbing light emitted from the organic EL element in the near-infrared to the visible range, performing wavelength distribution conversion, and radiating visible light comprising light at different wavelengths. A color filter layer has the functions of blocking light at specific wavelengths, and improving the color purity of visible light that has passed through a color conversion layer.
With respect to organic EL displays comprising a color conversion layer and a color filter layer, the following technologies have been disclosed as examples for realizing multicolor light emission.
For example, an organic EL display has been proposed, having a plurality of independently controllable organic EL elements, each comprising a transparent substrate, a color filter layer formed by evaporation deposition of a pigment and/or organic dye, a fluorescent conversion layer (equivalent to a color conversion layer) which performs conversion into light of a prescribed wavelength, and at least one organic light emission layer between two electrodes at least one of which is transparent (see Patent Document 1: Japanese Patent Application Laid-open No. 2002-75643). In this technology, the fluorescent light emission layer comprises at least one type of color conversion material, which absorbs light at short wavelengths and performs conversion into light at long wavelengths.
Further, as a method of forming a fluorescent conversion layer or color conversion layer, a method in which a liquid comprising a color conversion material dispersed in a resin is applied, and a method in which a color conversion material is deposited by evaporation deposition, sputtering, or another dry process, have been disclosed (see Patent Document 1, Patent Document 2: Japanese Patent Application Laid-open No. 2003-217859, and Patent Document 3: Japanese Patent Application Laid-open No. 2000-230172).
In general, when realizing a high-resolution color display using a color conversion-type organic EL display, it is effective to raise the concentration of the color conversion material in the color conversion layer, increase the absorptance of absorbed light in the color conversion layer, and obtain a high converted light intensity.
However, if the concentration of color conversion material is raised, a phenomenon called concentration quenching occurs, in which energy absorbed from light emitted from the organic EL element is repeatedly transferred between molecules in the color conversion material, and there is deactivation of the color conversion material, without accompanying light emission. In order to suppress concentration quenching, it is essential that the color conversion material be dissolved or dispersed in some medium to lower the concentration (see Patent Document 3). On the other hand, if the concentration of the color conversion material is lowered, absorptance of the light to be absorbed is reduced, and adequate converted light intensity is not obtained.
With respect to this problem, the film thickness of the color conversion layer is being increased to raise the absorptance and maintain color conversion efficiency. When a color conversion layer is used with the film thickness increased to approximately 10 μm in this way, there exist such problems as breakage of lines in electrode patterns at step portions, the difficulty of attaining high resolution, and, when combined with organic EL elements, changes in the properties of the organic EL layer due to residual water content or solvent in the color conversion layer, display defects, and similar. In addition, from the standpoint of reducing dependence on the viewing angle, increasing the film thickness of the color conversion film is not preferable.
In response to such conflicting demands, technology has been proposed for using evaporation deposition to form a host-guest color conversion layer having a film thickness of 2000 nm or less, as technology to provide a color conversion layer from which adequate converted light intensity is obtained without increasing the film thickness.
When forming the color conversion layer by evaporation deposition, the color conversion layer is formed over the entirety of the layer serving as the base, and so separation of the regions emitting light in each of the three primary colors (red, green, and blue) is not possible. Hence some means must be employed to form a color conversion layer having a fine pattern (subpixels) corresponding to specific primary colors. An application method using a metal mask is known as an example of a method of forming a thin film in a pattern in evaporation deposition (See Patent Document 1).
However, the properties of a metal mask necessitate penetrating openings, and so in order to secure strength for the metal mask, adequate intervals between adjacent openings, that is, frame width, must be secured. Consequently there is a limit to the fineness of the metal mask. Further, because wraparound of evaporated material behind the metal mask occurs, there is a limit to the fineness of the pattern formed using a metal mask. In actuality, the limit is a resolution level of 150 pixels per inch (ppi), and formation of patterns at resolutions exceeding this is difficult. Further, as substrates grow larger there are greater demands on the rigidity of the metal mask, and when rigidity is insufficient, mask bending poses a problem. Because of this problem also, it is difficult to increase substrate sizes. In addition, reductions in yield occur due to pattern shifting when placing the metal mask on the substrate for film deposition and other problems, and so there is the further problem that cost reduction is difficult.
In order to address problems with evaporation deposition methods described above, technology has been proposed in which an inkjet method is used to form a color conversion layer (see Patent Document 4: Japanese Patent Application Laid-open No. 2004-253179, Patent Document 5: Japanese Patent Application Laid-open No. 2006-73450, Patent Document 6: Japanese Patent Application Laid-open No. 2006-32010, and Patent Document 7: Japanese Patent Application Laid-open No. 2003-229261).
On the other hand, a method has been proposed for superposing a plurality of types of color filter layers to form light blocking portions in desired regions, with the aim of suppressing increases in the number of manufacturing processes and increases in manufacturing costs in color filter-type displays not accompanied by color conversion layers (designs in which, by passing only specific wavelength regions of the light source light, light having the desired frequencies is emitted to the outside) (see Patent Document 8).
Patent Document 8: Japanese Patent Application Laid-open No. 2004-94236
However, proposals to form color conversion layers using inkjet methods all employ barrier walls that are formed separately. A barrier wall is deemed necessary in order to prevent the spreading from their desired positions of liquid drops, adhering to the film deposition substrate through use of the inkjet method. Separate formation of barrier walls entails an increase in the number of manufacturing processes, and a consequent increase in manufacturing costs.
Further, when forming a color conversion layer and color filter layer with high resolution, markers for positioning must be provided. However, the separate formation of markers also entails an increase in the number of manufacturing processes, and a consequent increase in manufacturing costs.